A study of the ecological relationships and taxonomic status of two species of the genus Calanus (Crustacea: Copepoda)

• Main Author: Woodhouse, Charles D.
• Format: Text
• Language: English
• Published: University of British Columbia 1971
• Subjects: Arctic; Glacier Bay; Pacific; Indian; Davidson; Calanus glacialis; glacier; Alaska
• Online Access: https://dx.doi.org/10.14288/1.0093285; https://doi.library.ubc.ca/10.14288/1.0093285

This thesis presents the results of an investigation on the relationships between populations of closely related animals under apparent sympatric conditions. The mechanisms found have particular application toward understanding the species problem among members of the free-swimming marine copepod genus Calanus that possess a toothed inner surface on the coxopodites of the fifth pair of swirnming legs. The investigation describes the morphology, distribution, and general ecology of two forms of toothed Calanus from the far eastern North Pacific Ocean. Morphological differences were established and used to distinguish both forms on the oasis of length, shape of the anterior surface of the cephalothorax, proportionate differences in segments of the urosome and fifth swimming legs, and by the degree of asymmetry in the fifth pair of swimming legs of males. An additional feature was the length of a small spine on the fifth swimming legs of both forms. A general account of the distribution and ecology of both forms from Glacier Bay, Alaska, to the Mexican Border was derived from data gathered during several long cruises. The Large Form was found from Glacier Bay, Alaska, to Cape Mendocino, California. The Small Form was found from the Mexican Border to the vicinity of Vancouver Island, British Columbia. Along the outer coast, the Large Form appeared to be associated with Pacific Sub-Arctic water typical of the California Current, whereas the Small Form appeared to be associated with the warmer more saline water typical of Equatorial Pacific water associated with the Davidson Counter Current. A detailed analysis of the ecological relationships of both forms in a region of overlap was performed in Indian Arm, an inlet near Vancouver, British Columbia. In this inlet, the Large Form was generally associated with the cooler more saline deep water of the inlet. The Small Form occurred at shallower depths. Overlap between the populations of both forms was limited to Large Form females that rose to shallower depths during part of the year occupying nearly the same portion of the water column as the Small Form population. The yearly cycles of both forms in Indian Arm were shown to be different indicating different times of breeding for Large and Small Forms. On the basis of morphology and previous descriptions for toothed members of the genus Calanus, the Large Form appeared to be Calanus glacialis and the Small Form C. pacificus californi-cus. Based on the results of the distributional study and the ecological study, it was concluded that both forms were behaving as good species since separation of breeding populations both spatially and temporally appeared to be real, and the likelihood of interbreeding appeared to be small. In the classical sense, the two species are sympatric because their ranges overlap, and there is a strong indication that interbreeding occurs infrequently if at all. Association to different types of water and differences in yearly cycles appear to be the primary mechanisms that act to maintain the integrity of sympatric species. The vertical as well as horizontal space must be given equal consideration in planktonic studies. Under these conditions, therefore, the toothed Calanus spp. of Indian Arm are allopatric with respect to the water column.